keep a straight face—(also: keep one’s face) refrain from showing one’s feelings by one’s facial expression:
- All I felt called upon to do was to keep a straight face and pretend that everything was kosher.
keep straight—(also: go straight) be virtuous or well-behaved; refrain from criminal activities:
- She meant to marry him in two or three years, if he proved he could keep straight in the meanwhile.